Cardiac arrhythmias constitute one of the major causes of mortality and morbidity in the U.S. and present an enormous burden to public health. While current therapies have proven symptomatic and mortality benefit, there are a number of drawbacks with each leading to significant morbidity. Pharmacological approaches generally lack cellular and temporal precision, and while electrode-based approaches have better temporal resolution, they still lack cell specificity. Within the last several years, the Stanford Cellular-Optical Interface Laboratory (COIL) has successfully developed a system of genetically targeted, temporally precise optical control of cell firing utilizing two single-component ion channels with submillisecond opening kinetics in response to light stimuli, specifically Channelrhodopsin-2 (ChR2) and halorhodopsin (NpHR). This technique has allowed for functional control of specific neuronal cell types in vivo, with mouse models successfully demonstrating optical control of movement and sleep regulation. With regard to cardiac arrhythmias, the ability to locally activate specific regions or cell types within the heart would give rise to a new horizon of therapeutic options, from inhibiting propagation of arrhythmias, to pacing more physiologically, to altering sympathetic and parasympathetic input to the heart. Thus far, this technique has been demonstrated reliably in neurons, however has not been applied to other populations of excitable cells. The specific aims of this project are as follows: 1) To demonstrate genetically targeted, temporally precise optical activation of myocyte firing with ChR2. 2) To demonstrate genetically targeted, temporally precise optical inhibition of myocyte activity with NpHR. Furthermore, with co-expression of ChR2 and NpHR, we predict that bidirectional optical control of myocyte membrane potential can be achieved within the same cell. 3) To demonstrate that expression of ChR2 and NpHR will have minimal effects on the basic electrical properties and survival of cultured myocytes. Lentiviral gene delivery will be used to achieve myocyte expression of ChR2 and NpHR. Electrophysiological data will be gathered by whole cell patch clamp technique to demonstrate both activation and inhibition of single spikes as well as spike trains elicited by pulsed light. As the elderly population grows, the incidence of arrhythmias is projected to increase by three fold in the next fifty years, presenting a significant economic, social, and public health burden. By successfully demonstrating the fundamental principle of optogenetics at the cellular level, the possibilities are open to the next stage of development in bringing this technology closer to clinical application, with enormous potential to develop superior therapeutic options for cardiac arrhythmias.